1. Field of the Invention
This invention relates to an optical-thin-film material used to coat lenses, mirrors and prisms used in optical instruments such as cameras and video cameras or to coat optical devices such as optical disks. It also relates to an optical device provided with an optical thin film formed by coating such a material.
2. Related Background Art
In cameras, video cameras and other optical instruments, metal oxide thin-film coating is utilized in their anti-reflection films, filters, mirrors and so forth in order to enhance or reduce the reflection of light or in order to absorb or transmit specific wavelength light.
Of these metal oxide optical thin films, in particular, multi-layer thin films alternately superposingly formed of thin films having a high refractive index and thin films having a low refractive index can selectively transmit or reflect specific wavelength light. Hence, they are used as a coating material for optical filters or prisms in a large number in optical instruments as typified by projectors. Such multi-layer thin films are produced by physical vapor deposition (PVD) as typified by vacuum deposition (or evaporation) and sputtering. These multilayer thin films, however, have so large a total number of films that it often takes a long time for film formation, leading to a problem of a low productivity. As a countermeasure therefor, the designing of multi-layer thin films is elaborated to lessen the total number of films. As other measures, in vacuum deposition, films are formed at a high rate or materials are used which can enjoy a short premelt time for stabilizing evaporation sources. In sputtering, magnetron sputtering is employed, which is used to form films at a higher rate. Also, in order to improve productivity, film-forming systems are being shifted from batch-type systems to continuous-type systems.
In the case of the continuous-type systems in vacuum deposition, films are formed while supplying substrates and replenishing evaporation source targets. Hence, the evaporation source targets must be those having shapes suited for individual systems and also can attain stable characteristics during continuous use. In general, when the multi-layer thin films alternately superposingly formed of high-refractive-index films and low-refractive-index thin films are formed, materials that may cause variations in refractive index because of compositional variations or materials that have much non-homogeniety are not preferable as evaporation substances when the multi-layer thin films are formed by the continuous-type systems.
When these filters and prisms coated with the coating material are used as beam splitters in optical instruments, optical multi-layer thin films are often face to face bonded with adhesives when used. As the adhesives, ultraviolet-curable resins are used in view of an advantage that they can cure in a short time and a solvent need not be used. Widely used is a method in which the ultraviolet-curable resins are irradiated by ultraviolet light to cure and join the thin films. However, such ultraviolet-light irradiation causes the metal oxide optical thin films to undergo deoxydation reaction due to photochemical reaction, brining about a problem of a deterioration of optical thin films, e.g., a decrease in light transmission.
As a material that may not cause any compositional variations or have any non-homogeniety and has a high refractive index, a compound represented by the formula: La.sub.2 Ti.sub.2 O.sub.7-x (wherein x is 0.3 to 0.7) is known in the art as disclosed in Japanese Patent Application Laid-Open No. 6-235803.
However, in deposition materials conventionally used for optical thin films having a high refractive index as stated above, the materials may so greatly scatter that it takes a long premelt time for stabilizing evaporation sources, or their continuous use may cause compositional changes, resulting in a change in refractive index. Also, because of a certain non-homogeniety, the multi-layer formation of films may result in a difference of values from designed values. Also, the irradiation of deposited thin films with ultraviolet light may cause an increase in absorptivity, which inhibits the bonding of the thin films using ultraviolet-curable resins. There are other various problems.
In the compound represented by the formula La.sub.2 Ti.sub.2 O.sub.7-x (x=0.3 to 0.7), too, although it is a material that may not cause any compositional variations or have any non-homogeniety and has a high refractive index, this material may so greatly scatter at the time of premelting that it must be premelted for a long time every time materials are added. Moreover, it may cause a deterioration due to irradiation with ultraviolet light. Thus, this material is not a preferred material.
As materials for optical thin films, substances or compounds are required which have a refractive index intermediate between that of aluminum oxide (Al.sub.2 O.sub.3), having a refractive index of about 1.65, and that of zirconium oxide (ZrO.sub.2), having a refractive index of about 2.0. Such substances or compounds include silicon monoxide (SiO), magnesium oxide (MgO), lead fluoride (PbF.sub.2), a mixture of aluminum oxide (Al.sub.2 O.sub.3) and zirconium oxide (ZrO.sub.2), disclosed in U.S. Pat. No. 3,934,961, and a mixture of lanthanum oxide (La.sub.2 O.sub.3) and aluminum oxide (Al.sub.2 O.sub.3), disclosed in Japanese Patent Application Laid-Open No. 6-184730, which have been put into use.
However, when used as optical thin films, the above substances or compounds put into use as being achievable of the intermediate refractive index (about 1.6 to 1.9) have problems respectively as stated below. Thus, any substances or compounds suited as optical thin films having an intermediate refractive index are substantially almost unavailable.
That is, the silicon monoxide (SiO) has a problem that thin films cause an absorption to result in a decrease of transmittance. The magnesium oxide (MgO) has a problem that it reacts with water or carbon dioxide in the air to change into magnesium hydroxide or magnesium carbonate to cause clouding. The lead fluoride (PbF.sub.2) has a problem of environmental pollution due to lead.
The mixture of aluminum oxide (Al.sub.2 O.sub.3) and zirconium oxide (ZrO.sub.2), though having a good reproducibility of refractive index, has a problem that it may cause a compositional deviation in materials because of a difference in vapor pressure to make it unable to carry out cost-advantageous continuous deposition while replenishing materials. The mixture of lanthanum oxide (La.sub.2 O.sub.3) and aluminum oxide (Al.sub.2 O.sub.3) has a problem that the lanthanum oxide in the mixture reacts with water in the air to change into lanthanum hydroxide to cause expansion, making sintered evaporation source materials and formed films break down. Their breaking down into powder causes fine particulate matters because of chemicals which may scatter when dissolved, to further cause a problem of compositional deviation in film materials during the deposition.
As stated above, although various deposition materials are conventionally used as substances or mixtures achievable of the refractive index intermediate between alumina (about 1.65) and zirconia (about 2.0), they have properties undesirable as deposition materials, causative of compositional deviation or occurrence of absorption during the deposition.